As data transmission rates in multiple-input-multiple-output (MIMO) systems increase, bandwidth and constellation sizes of these systems also typically increase. As bandwidth increases, noise has a larger impact on the performance of these systems. Near end crosstalk (NEXT) and far end crosstalk (FEXT) are two types of noise in MIMO systems. NEXT is crosstalk coupling between a local receiver of one channel and local transmitters of the other channels. FEXT is crosstalk coupling between a local receiver of one channel and remote transmitters of the other channels.
In the next generation 10GBASE-T standard, channel bandwidth will be in the range of 400-500 MHz as compared to 62.5 MHz in 1000BASE-T. Since FEXT coupling increases with frequency, FEXT noise will become more significant at this higher operating frequency and should be mitigated to ensure reliable data transmission.
FEXT noise arises from the coupling of other transmission lines into the main line. The signal from the main channel is usually much stronger than the signal from FEXT. The model for FEXT as given by ISO 11801 channel specification is:
Pair-to-pair ELFEXT:CAT 5e−20log10(10(63.8−20log10ƒ)/−20 + 4 · 10(75.1−20log10ƒ)/−20),1 ≦ ƒ ≦ 100CAT 6−20log10(10(67.8−20log10ƒ)/−20 + 4 · 10(83.1−20log10ƒ)/−20),1 ≦ ƒ ≦ 250
Power-Sum ELFEXT:CAT 5e−20log10(10(60.8−20log10ƒ)/−20 + 4 · 10(72.1−20log10ƒ)/−20),1 ≦ ƒ ≦ 100CAT 6−20log10(10(64.8−20log10ƒ)/−20 + 4 · 10(80.1−20log10ƒ)/−20),1 ≦ ƒ ≦ 250The models can be extended to the frequencies above those specified in the standard specifications. The models show that the FEXT coupling increases as the frequencies increases.
Referring now to FIG. 1, first and second exemplary network devices 10 and 12, respectively, communicate over a data communications medium with four full-duplex channels 14-A, 14-B, 14-C, and 14-D. For example, the first and second network devices 10 and 12, respectively, may be Gigabit or 10G Ethernet network devices. Each of the channels 14 at the first and second network devices 10 and 12, respectively, are identified as A, B, C, or D and include a transceiver 16 and a hybrid 18. The transceivers 16 include transmitters 20 and receivers 24, which process transmitted and received data, respectively. The hybrids 18 facilitate full-duplex communications over the data communications medium.
FEXT occurs at a local receiver due to signals transmitted by remote transmitters associated with other channels. FEXT is schematically shown at 26, 28 and 30. FEXT 26 is crosstalk coupling between signals generated by the transmitter of transceiver 16-B2 and signals received at the receiver 24-A1 of transceiver 16-A1. FEXT 28 is crosstalk coupling between signals generated by the transmitter of transceiver 18-B2 and signals received at the receiver 24-A1 of transceiver 16-A1. FEXT 30 is crosstalk coupling between signals generated by the transmitter of transceiver 18-C2 and signals received at the receiver 24-A1 of transceiver 16-A1. Similar FEXT occurs in Channels B, C and D.